A conventional boiling water nuclear reactor includes a reactor core disposed inside a pressure vessel and spaced above a lower head thereof to define a lower plenum. Disposed above the core is a steam separator followed in turn by a steam dryer. The vessel is partially filled with water above the reactor core, which water is conventionally circulated through an annular downcomer radially surrounding the core and into the lower plenum wherein it is turned upwardly through the core and heated therein for generating steam.
A plurality of conventional control rod drives are typically disposed below the lower head of the pressure vessel and have drive rods extending upwardly through the lower head and the lower plenum to the reactor core. The drive rods are joined to conventional control rods by conventional bayonet-type couplings which allow individual control rods to be removed. The control rod drives are effective for retracting the drive rods to retract the control rods completely from the reactor core and into the lower plenum. The drive rods may also be extended for positioning the control rods fully into the core between adjacent fuel bundles. Accordingly, the lower plenum must have suitable height for fully receiving the control rods when retracted.
A natural circulation boiling water reactor includes an annular chimney extending from the core upwardly to the conventional steam separators. The chimney provides open space which could be used for retracting the control rods and, thusly, allowing the pressure vessel to be made correspondingly shorter. In the conventional boiling water reactor, the control rods are retracted into the lower plenum, and, therefore, the lower plenum must have sufficient height for containing the retracted control rods. If the control rods could instead be retracted upwardly above the reactor core into the chimney, the lower plenum could be made substantially shorter. In turn, the reactor core could be positioned lower within the pressure vessel, thusly requiring less water inventory within the pressure vessel to keep the core covered with water after a conventional blowdown occurrence.
However, in order to retract the control rod upwardly from the reactor core when the control rod drives are disposed below the reactor core, the coupling between the control rod and the control rod drives must necessarily pass between adjacent fuel bundles within the reactor core. The coupling must be mechanically secure and should allow for replacement of individual control rods as desired. A conventional coupling consists of a bayonet assembly which allows coupling and decoupling of the control rod from the control rod drives. However, the bayonet assembly is relatively large in diameter, and is larger than the spacing between adjacent fuel bundles in a conventional reactor core which therefore prevents retraction of the control rods upwardly above the reactor core.